Knowledge of the path and progress of instrumented pipeline-inspection tools (“pigs”) flowing with product through pipelines is highly desired, especially if the pig becomes stuck in the pipeline. It is common for a pig to carry a portable transmitter or magnets which allow its passage to be detected by instruments placed in the Right of Way on the ground surface above the pipeline. These tools are referred to herein as Above Ground Markers (AGMs). Pigs with portable transmitters may emit a continuous signal, or may emit a pulsed signal. One skilled in the art would appreciate that other variations may also occur in the transmitter signal which can be accounted for in the detection of the pigs to distinguish the transmission from noise, and improve identification.
The type of transmitter carried by a pig is determined by the purpose of the pig. When timing accuracy is important a continuous transmitter is desired, as it allows for a clearer null signal as the transmitter passes under an AGM. Similarly magnetic signals can be utilized for precise timing as they generate a clear zero crossing as the pig passes under the AGM. Precise timing is necessary to correlate the data collected by the pig's instruments with known time stamps and geographic locations. Therefore, drifts and other “dead reckoning” errors from the pig's onboard inertial navigation instruments can be adjusted to precisely determine the location of ‘defect’ conditions recorded by the pig.
Pulsed signals may be more desirable for instances where precise timing is not required, such as cleaning pigs. Cleaning pigs do not normally collect and record data in the pipeline, so the reason for tracking them is to ensure they are properly progressing through the system, and identify their specific location if they get stuck. Since it may take time to locate a stuck pig, the pulsed signals are desirable as they give longer running times than continuous transmitters given a specific amount of battery life. E.g., A pulsed signal pig with a 50:50 duty cycle is expected to operate for approximately twice as long as a continuous signal pig with the same battery life.
AGMs detect and report electronic signals from the pig as it passes through the pipeline. As the pig is passing through a pipe, the pipe's construction may shield its transmission signal. The pipe may also be buried under more than twenty feet (20′) of ground. This attenuation of the transmission signal means the AGM must be very sensitive. This sensitivity results in many false signals being detected which may be mistaken for the signal of the passing pig.
Typically, human operators must be on numerous sites along the pipeline to monitor these instruments and report the pig's progress (or lack thereof). This process of monitoring the pig so that its location can be approximated in the case it becomes stuck involves significant expense.
Some instrument packages exist that contain additional electronics which allow automatic unattended recording of the detected signals and thus the efficient transmission of these results to remote observers is desirable. However, due to the remote locations of most pipelines, transmission must be accomplished by satellite links which can be an expensive transmission medium.
The most cost-effective satellite links allow only a very limited amount of data per transmission, usually single packets of less than sixty (60) bytes, and rarely more than a few hundred bytes. For this reason, it has been a common practice to transmit a standard identification message when the pig passage event has occurred. No actual data of the event is transmitted by these systems. Rather, they rely purely on the time stamp applied to a message when the satellite ground server receives and forwards the standard message. This data packet is transmitted three times in approximately seven and a half minute increments to account for gaps in satellite coverage which may prevent a single data transmission from being received. The data is relayed by a satellite to a ground station where the message is forwarded by other communications means to a final destination. A common means of forwarding the data is via the Internet in the form of e-mail or Short Message Service (SMS) Text Messages.
Depending on which of the three transmitted data packets was received and forwarded, the server time stamp may differ from the actual pig passage time by as much as fifteen (15) minutes. When a pig is being utilized to determine flow rates, this delay can result in a significant error in calculations. Environmental conditions are often detected by the AGMs, and can trigger the AGM in the same manner as a passing pig. Transmitting a signal each time a triggering event occurs without any further information can result in false transmissions causing remote monitors to believe a pig has passed when such an event has, in fact, not occurred.
The current method of remote monitoring is to estimate when a pig passage is anticipated, and watch for a signal to arrive approximately at the anticipated time. If a signal is received, then the remote monitor assumes it was the signal of the pig passage. If more than one is received, then the one closest to the estimated arrival time is assumed to be the correct signal and the others are rejected as false signals. These assumptions result in a lot of unreliable “guess” work which is costly to the industry.